Biotite as a petrogenetic discriminator in the polymetamorphosed rocks: A case study from the Champaner supracrustal rocks, Western India

Abstract

The mineral chemistry of Fe-Mg biotite has served as an aid to the petrogenesis of igneous rocks, especially for identifying the parent magma compositions. Though the interpretation of igneous and metamorphic processes utilizing biotite mineral chemistry is not upfront, it certainly provides preliminary insights into the petrogenetic characteristics of rocks. Apart from thermometry and chronology, studies emphasizing on the significance of biotite mineral chemistry for unveiling the protolith compositions, grade of metamorphism, origin, and P-T conditions of metamorphic rocks are poorly documented in the literature. Here we employ biotite mineral chemical compositions i.e., EPMA (electron probe micro-analyzer) data from the metapelitic rocks of the Meso-Neoproterozoic Champaner Group, exposed as an arcuate fold belt in western India to unravel the metamorphic rock record. This group of rocks display a complex deformational and polymetamorphic history, evident in distinct mineral assemblages that reflect regional, contact, and combined regional-contact metamorphism. The field observations, mesoscopic features of rocks, reaction textures, and biotite petrographic characteristics have revealed three distinct varieties of metapelitic rocks i.e., phyllites, spotted phyllites, and hornfelses. The mineral chemistry and cation substitution mechanisms of biotite selected from phyllite, spotted phyllite and hornfels suggests, the chemical composition of biotite in these rocks is a function of the bulk rock composition, co-existing minerals, type, and grade of metamorphism and thereby attesting to the pressure-temperature conditions of rock. Further, various compositional classification diagrams and Ti-in biotite thermometry demosntrate an increase in the grade of metamorphism from west to east of Champaner Group with intermittent as well as late-stage plutonic felsic intrusions. Thus, biotite being frequently the dominant or sometimes even sole Fe-Mg mineral with temperature-pressure driven substitutions in low-grade metamorphic rocks can offer a unique opportunity to unlock valuable insights into their metamorphic history.